If you don't mind a refrigerator sized sub, you can have excellent SQ and ridiculous output, on an extremely reasonable budget.
If I did this, I'd have a friend help me.
The Goat Fainter
aka Tuba folded horn sub. $15 for plans. And after asking Matt, he said roughly $275 outside of the existing amp. Then he said that it outperformed his
dual Kappa Perfect builds, in both output and SQ.


Mind you, a single Kappa build will already blow away anything that is probably on your radar.
The Kappa build is a sticky in the DIY subforum . . .
I've never built any sub or speaker, but if I did it all over again today . . .